Dystonia is a movement disorder that affects at least 70,000 people in the UK, and in about a third of these, the disorder will start during childhood.
The main clinical feature of dystonia is posturing of the affected body part caused by abnormal involuntary muscle activity.
There is typically a shifting pattern of agonist and antagonist contraction pulling across a joint, which causes a rather mobile abnormal posture combined with slow writhing movements sometimes known as athetosis.
Dystonia is often task or action specific, for example it may only occur when writing.
It may be relatively absent at rest but dramatically present during action. This action specificity can also be seen when dystonia affects walking - patients may have great difficulty walking forwards, but will be able to walk backwards easily.
Some patients also have a sensory trick or 'geste', where a light touch on the body part affected may significantly correct the posture. Pain or discomfort may occur due to muscle spasm, but it is not usually the dominant clinical feature.
This article discusses the classification of dystonia, important causes in childhood and how they can be assessed and treated.
Classifying dystonia
The simplest classification of dystonia is based on the part of the body affected.
Dystonia is focal if it affects just one body part, segmental if it affects two adjacent body parts, hemidystonia if it affects just one side of the body or generalised if it affects multiple body parts, including the trunk. Dystonia affecting particular body parts is called blepharospasm (eyes) and torticollis/cervical dystonia (neck).
The most useful classification is into primary and secondary/degenerative causes. In primary dystonia, dystonia is the only clinical feature and other neurological and systemic symptoms and signs do not occur. There is no degeneration of the brain and although the dystonia may be disabling, it will plateau in severity after about one year of progression.
Primary dystonia has a distribution linked to age at onset. Primary dystonia presenting in children and teenagers will almost always affect the limbs and not the face, whereas adult (over 45 years) onset dystonia almost always affects the head and neck.
In secondary dystonia there is an external cause, such as stroke, brain injury or exposure to certain drugs (dopamine receptor blocking drugs).
In degenerative dystonia there can be multiple neurological (and sometimes systemic) signs and symptoms. The patient also experiences progression in severity of symptoms.
Primary dystonia
The most common cause of primary dystonia in children is called DYT1 dystonia. A genetic mutation in the DYT1 gene causes childhood onset of dystonia, usually in the foot or hand, which will gradually progress over a year to cause generalised dystonia. Some patients can remain with just focal dystonia.
The condition will then plateau, but disability can be severe. It is an autosomal dominantly inherited condition, but about 60 per cent of people who carry the gene mutation never develop symptoms.
Dopa-responsive dystonia
This rare type of dystonia presents in childhood. It is important because it is effectively curable by the administration of levodopa. Typically patients present in childhood with a combination of dystonia (often affecting the legs) and parkinsonism.
Other features may occur, such as apparent spasticity, brisk reflexes, upgoing plantar responses and a diurnal fluctuation of symptoms with gradual worsening as the day goes on.
There are numerous cases of patients with dystonia being misdiagnosed as cerebral palsy. It is therefore essential that unless there is a clear reason for a child to have dystonia, a trial of levodopa is given. Diagnosis is possible using genetic and metabolic tests.
Secondary dystonia
The commonest cause of secondary dystonia in children is birth hypoxia. Such patients often have a combination of spasticity and weakness with dystonic writhing movements, and the term athetoid cerebral palsy is often used.
It is important not to dismiss dystonia in a child as cerebral palsy without appropriate investigation. One would not expect symptoms to progress over time - if this occurs, a re-evaluation of the diagnosis is indicated.
Degenerative dystonia
There are numerous genetic and metabolic conditions that present in childhood with dystonia. In these conditions, dystonia is usually only part of a wider neurological syndrome which is relentlessly progressive over time.
Diagnosis is often complex and needs specialist evaluation. Correct diagnosis is important as some causes, for example Wilson's disease, are treatable.
Assessment and management
A barrier to correct diagnosis and management of dystonia is that the condition is often not recognised and classified as something else (for example, cerebral palsy or psychogenic).
Diagnosis and management can be complex and needs specialist help from a paediatrician or paediatric neurologist, but the crucial first step is the recognition of the disorder and appropriate referral.
Treatment relies on three main tools: medication, such as anticholinergics; botulinum toxin injections, which can help focal dystonia; and deep brain stimulation surgery.
This last option has recently emerged and can be effective for patients with primary dystonia. It is much less effective for other forms of dystonia, but still may be appropriate in highly selected cases.
Long-term care
Many patients with dystonia will have chronic symptoms which progress over time. Prompt specialist referral for diagnosis and treatment is vital, as is ongoing management of care needs in the community.
These needs can be complex in the setting of generalised and degenerative dystonias, and often a multidisciplinary rehabilitation approach is best with input from the specialist neurology team.
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- Dr Edwards is National Institute for Health Research clinician scientist at Institute of Neurology UCL, and honorary consultant neurologist at the National Hospital for Neurology and Neurosurgery.
